The invention relates to a method for modulating an output voltage of a transmitter circuit, and a transmitter circuit, in particular for Bluetooth and Hiperlan applications, comprising a voltage controlled oscillator having a tank circuit, a digital/analog converter and an antenna circuit.
In some power amplifier applications, a constant and envelope modulation is necessary. This is done with an upmixer and a power amplifier. For example, a transmitter circuit for transmitting RE signals is conventionally constructed around the concept of an upmixer and a power amplifier. A voltage controlled oscillator directs a signal to a first port of the upmixer, the second port of the upmixer receives the IF or baseband signal. The upmixer multiplies the two signals, resulting in the RE signal which is then sent to the power amplifier. After being amplified by the power amplifier, the signal is past on to the antenna circuit. The upmixer can be replaced by a phase-locked loop, hereinafter (xe2x80x9cPLL-loopxe2x80x9d) wherein a fractional N partitioner is modulated by the signal to be sent. In this manner, the voltage controlled oscillator is modulated as well, and the modulated signal is sent to the antenna circuit.
This concept for a transmitter circuit can be used as well in applications with a low output power, such as 0 dBm. Examples for such applications are Bluetooth and Hyperlan. The disadvantage is, however, that the efficiency of the power amplifier is low in these applications. This is due to the fact, that the power used in the power amplifier in order to obtain such low output power is not low. In the Bluetooth application, the power amplifier consumes a current of 10 to 15 mA.
In view of the above, it is an object of the invention to provide a method for modulating an output voltage of a RF transmitter circuit and a RF transmitter circuit meeting the standard of the envisioned application and having a lower power consumption as compared to the none methods and circuit respectively for this purpose.
To achieve the above object, a method is provided for modulating an output voltage of a RF transmitter circuit, in particular for Bluetooth and Hiperlan applications, comprising a voltage controlled oscillator, a digital/analog converter and an antenna circuit, the method comprising sending an output signal from the voltage controlled oscillator directly to the antenna circuit and directly modulating a frequency of the output signal of the voltage controlled oscillator. By using this method, the upmixer or franctional N partitioner and the power amplifier used in the state of art is not necessary. The power consumption of the overall circuit can significantly be reduced.
In a preferred embodiment of the method of the invention a tank circuit of the voltage controlled oscillator is capacitively loaded for direct modulation of the frequency of the voltage controlled oscillator. Thereby, the modulation of the frequency of the voltage controlled oscillator is carried out in a most effective way.
In a further preferred embodiment of the method of the invention an output of the digital/analog converter is directly fed to the voltage controlled oscillator for capacitively loading the tank circuit of the voltage controlled oscillator. As the digital analog converter translates the digital base band signal into a capacitive load, the modulation can be realized by adding or removing this load to or from the tank in order to effect the required frequency changes of the modulation.
In a further preferred embodiment of the method of the invention a crystal oscillator circuit of the voltage controlled oscillator is capacitively loaded for direct modulation of the frequency of the voltage controlled oscillator. An alternative way to introduce the desired modulation in the voltage controlled oscillator is to capacitively loaded a crystal oscillator circuit usually provided as a reference source for the center frequency of the voltage controlled oscillator.
In a further preferred embodiment of the method of the invention an output of the digital/analog converter is directly fed to the crystal oscillator circuit of the voltage controlled oscillator for capacitively loading the crystal oscillator circuit of the voltage controlled oscillator. In a most advantages way, the voltage controlled oscillator can be modulated by tuning of the frequency of the crystal oscillator circuit via the capacitive loading of the digital/analog converter. The voltage controlled oscillator follows the crystal oscillator via the closed PLL loop.
In a further preferred embodiment of the method of the invention a constant amplitude modulation, in particular GFSK (Gaussean frequency shift keying) or GMSK (Gaussean medium shift keying), is used. These specific modulation methods a particularly suitable for the required modulation in Bluetooth and Hiperlan applications.
To achieve the above object, a RF transmitter circuit, in particular for Bluetooth and Hiperlan applications, comprises a voltage controlled oscillator, a digital/analog converter and an antenna circuit, wherein the voltage controlled oscillator is adapted to send an output signal directly to the antenna circuit and wherein the digital/analog converter is arranged to modulate an output frequency of the voltage controlled oscillator. In such a transmitter circuit the voltage controlled oscillator provides sufficient power to send its output signal to the antenna circuit directly whereby the necessity of an upmixer which is power consuming like the power amplifier, is avoided and the power consumption also to the lack of an additional power amplifier is reduced.
In a preferred embodiment of the transmitter circuit of the invention a capacitive load circuit is connected to the tank circuit of the voltage controlled oscillator for modulating the frequency of the voltage controlled oscillator. This is one of the two advantages ways to effect modulation of the voltage controlled oscillator.
In a further preferred embodiment of the transmitter circuit of the invention the digital/analog converter is connected to the voltage controlled oscillator for capacitively loading of the tank circuit of the voltage controlled oscillator. This circuit arrangement is advantages in that no addition circuit stages are provided in between the digital/analog converter and the tank circuit of the voltage controlled oscillator. Rather, the tank circuit is directly influenced by the digital analog converter.
In a further preferred embodiment of the transmitter circuit of the invention a capacitive load circuit is connected a crystal oscillator circuit of the voltage controlled oscillator for modulating the frequency of the voltage controlled oscillator. This is the second one of the two advantages ways of modulating the frequency of the voltage controlled oscillator.
In a further preferred embodiment of the transmitter circuit of the invention the digital/analog converter is connected to the crystal oscillator circuit of the voltage controlled oscillator for capacitively loading the crystal oscillator circuit of the voltage controlled oscillator. Here again, no additional circuitry is needed for modulating the output frequency of the voltage controlled oscillator.
In a further preferred embodiment of the transmitter circuit of the invention the crystal oscillator circuit of the voltage controlled oscillator is coupled to the voltage controlled oscillator via a PLL loop. The PLL loop assures that the voltage controlled oscillator follows the modulation of the crystal oscillator circuit.
In a further preferred embodiment of the transmitter circuit of the invention the PLL loop comprises a divider circuit, a phase detector circuit, and a loop filter circuit. This is an advantages circuit for the PLL loop to achieve the desired function of the PLL loop to assure that the output frequency of the voltage controlled oscillator follows the output of the crystal oscillator circuit.
In a further preferred embodiment of the transmitter circuit of the invention the voltage controlled oscillator comprises a center frequency setting circuit for tuning a center frequency of the voltage controlled oscillator. The center frequency setting circuit enables to tune the center frequency of the voltage controlled oscillator to different values within a certain range of frequency in a simple and advantages way.
In a further preferred embodiment of the transmitter circuit of the invention the center frequency setting circuit for tuning the voltage controlled oscillator comprises a tuning voltage source and a voltage controlled capacitor circuit connected to the tank circuit. This circuit arrangement enables in an advantages way to set the center frequency of the voltage controlled oscillator and also to couple any modulation which might be carried out at the side of the crystal oscillator of the voltage controlled oscillator to the tank circuit of the voltage controlled oscillator.
In a further preferred embodiment of the transmitter circuit of the invention the voltage controllable capacitor circuit connected to the tank circuit comprises two varactors connected as capacitors, a node between the capacitors being connected to the tuning voltage source. The use of varactors enables to embody the voltage controlled capacitor circuit in MOS technology in one piece with the rest of the transmitter circuit In a further preferred embodiment of the transmitter circuit of the invention the tank circuit is connected via a first resistance circuit to ground and via a second resistance circuit to a supply voltage source. This circuit arrangement enable to built the voltage controlled oscillator in an advantages way to have sufficient power to output the output signal directly to the antenna and to drive the antenna circuit accordingly.
In a further preferred embodiment of the transmitter circuit of the invention the first resistance circuit comprises N-type MOSFET devices, and the second resistance circuit comprises P-type MOSFET devices. By the use of such type of active devices, the negative resistance stages between the tank and ground and the tank and supply voltage are embodied in a most efficient way.
In a further preferred embodiment of the transmitter circuit of the invention the digital/analog converter comprises a bit voltage source and a capacitve modulator circuit. Preferably, the capacitive modulator circuit comprises 2n pairs of MOSFET devices, realizing variable voltage controlled capacitance devices, the nodes between the MOSFET devices of a pair forming output nodes of the digital/analog converter. The circuit arrangement of the digital/analog converter is a most efficient way to translate the bit input into an analog output while the output signals of the digital analog converter can be used either to modulate the output of the tank circuit of the voltage controlled oscillator or the crystal oscillator used in the voltage controlled oscillator.
In a further preferred embodiment of the transmitter circuit of the invention the antenna circuit comprises a series of capacitors connected between the outputs of the tank circuit, the capacitors connected between inputs of the antenna circuit being controllable with respect to the capacitance thereof. This circuit arrangement enables to match the antenna circuit or to compensate variations in the antenna aloud by adapting the impedance of the circuit.